Peripheral blood smear

Background

• The peripheral blood smear (PBS), also called a blood film, involves microscopic examination of a stained blood sample to evaluate cell morphology across all three cell lines (red blood cells, white blood cells, and platelets)
• Follows the complete blood count (CBC) as the second most performed test in the hematology laboratory
• Provides morphologic information that automated hematology analyzers cannot — including cell shape abnormalities, inclusions, parasites, and immature/atypical cells
• Stained using Wright or Wright-Giemsa stain; examined under light microscopy at 100× (oil immersion) for morphologic detail
• Collected in an EDTA (purple/lavender top) tube; smear should ideally be prepared within 2 hours of collection to minimize artifact
• Approximately 10–25% of CBC samples trigger a manual smear review based on abnormal automated flags or institutional criteria

Clinical Features

The PBS is a diagnostic tool, not a clinical condition. It is ordered when the clinical picture or CBC abnormalities suggest a diagnosis requiring morphologic confirmation. Key morphologic findings and their clinical associations are outlined below.

Red Blood Cell Morphology

Finding	Description	Clinical Association
Schistocytes	Fragmented RBCs (helmet cells, triangles, crescents)	TTP/HUS, DIC, HELLP, mechanical heart valves, malignant hypertension; ≥ 1% is suggestive of thrombotic microangiopathy (TMA)
Spherocytes	Small, round RBCs lacking central pallor	Hereditary spherocytosis, autoimmune hemolytic anemia (warm AIHA), transfusion reactions, burns
Sickle cells (drepanocytes)	Crescent/sickle-shaped RBCs	Sickle cell disease
Target cells (codocytes)	Bull's-eye appearance with central and peripheral hemoglobin	Thalassemia, hemoglobin C disease, liver disease, iron deficiency, post-splenectomy
Tear-drop cells (dacrocytes)	Drop-shaped RBCs	Myelofibrosis, bone marrow infiltration (myelophthisis), thalassemia major, megaloblastic anemia
Burr cells (echinocytes)	Evenly spaced short spicules with preserved central pallor	Uremia/renal failure, artifact (drying, EDTA); may be seen in liver disease
Acanthocytes (spur cells)	Irregularly spaced thorny projections	Severe liver disease (spur cell anemia), abetalipoproteinemia, post-splenectomy
Rouleaux formation	RBCs stacked like coins	Multiple myeloma, Waldenström macroglobulinemia, chronic inflammation, elevated ESR
Howell-Jolly bodies	Nuclear remnants (dark inclusions) within RBCs	Post-splenectomy, functional asplenia (e.g. sickle cell disease), megaloblastic anemia
Basophilic stippling	Aggregated ribosomal RNA appearing as blue dots	Lead poisoning, thalassemia, sideroblastic anemia, myelodysplastic syndrome
Heinz bodies	Denatured hemoglobin inclusions (require supravital stain)	G6PD deficiency (during hemolytic crisis), unstable hemoglobin variants, oxidant drug exposure
Polychromasia	Bluish-gray tint to RBCs (reticulocytes)	Active bone marrow response to anemia (hemolysis, acute blood loss)
Hypersegmented neutrophils	Neutrophils with ≥ 5 lobes (or > 5% with 5 lobes)	Vitamin B12 or folate deficiency (megaloblastic anemia)
Nucleated RBCs	RBCs with retained nuclei (normoblasts)	Severe anemia, hemolysis, myelophthisic processes (bone marrow infiltration), extramedullary hematopoiesis, neonates (normal finding)

White Blood Cell Morphology

Finding	Description	Clinical Association
Blasts	Large immature cells with high nuclear-to-cytoplasmic ratio, fine chromatin, prominent nucleoli	Acute leukemia (AML, ALL) — hematologic emergency requiring immediate consultation
Left shift	Increased immature granulocytes (bands, metamyelocytes, myelocytes)	Severe bacterial infection, sepsis, physiologic stress, myeloproliferative disorders
Toxic granulation	Prominent dark granules in neutrophil cytoplasm	Severe infection/sepsis, inflammation
Döhle bodies	Light blue cytoplasmic inclusions in neutrophils	Severe infection, burns, May-Hegglin anomaly
Toxic vacuolization	Cytoplasmic vacuoles in neutrophils	Severe sepsis/bacteremia (associated with high specificity for bacteremia)
Auer rods	Pink rod-shaped crystalline inclusions in blast cytoplasm	Pathognomonic for AML (especially APL/M3); hematologic emergency
Atypical lymphocytes	Large lymphocytes with abundant pale blue cytoplasm and scalloped borders ("Downey cells")	Infectious mononucleosis (EBV), CMV, other viral infections, drug reactions
Smudge cells	Fragile lymphocytes that rupture during smear preparation	CLL (when numerous)
Pelger-Huët anomaly	Bilobed neutrophils	Inherited (benign); acquired (pseudo-Pelger-Huët) in myelodysplastic syndrome
Hypersegmented neutrophils	≥ 5 nuclear lobes	Megaloblastic anemia (B12/folate deficiency)

Platelet Morphology

Finding	Description	Clinical Association
Platelet clumps	Aggregated platelets on smear	Pseudothrombocytopenia (EDTA-dependent clumping) — falsely low automated platelet count; re-draw in citrate tube to confirm
Giant platelets	Platelets approaching or exceeding RBC size	Bernard-Soulier syndrome, May-Hegglin anomaly, ITP (young active platelets), myeloproliferative disorders
Decreased platelets on smear	< 1 platelet per oil-immersion field	True thrombocytopenia (each platelet per OIF ≈ 10,000–15,000/μL)

Infectious Organisms

Finding	Clinical Association
Malaria parasites (intra-erythrocytic rings, gametocytes, schizonts)	Malaria — thick smear for detection, thin smear for speciation; gold standard diagnostic test
Babesia (intra-erythrocytic ring forms, Maltese cross/tetrad)	Babesiosis — may resemble Plasmodium falciparum
Anaplasma/Ehrlichia morulae (intracytoplasmic inclusions in granulocytes or monocytes)	Anaplasmosis/Ehrlichiosis
Borrelia spirochetes (rare)	Relapsing Fever
Trypanosomes	Trypanosomiasis

Differential Diagnosis

Conditions Diagnosed or Strongly Suggested by PBS

• Thrombotic microangiopathy (TTP/HUS/DIC): Schistocytes + thrombocytopenia
• Acute leukemia: Circulating blasts; Auer rods pathognomonic for AML
• Malaria/Babesiosis: Intra-erythrocytic parasites
• Sickle cell disease: Sickle cells on smear
• DIC: Schistocytes + thrombocytopenia + abnormal coagulation studies
• Myelofibrosis/Marrow infiltration: Tear-drop cells + nucleated RBCs + left shift (leukoerythroblastic picture)
• CLL: Absolute lymphocytosis + smudge cells
• Megaloblastic anemia: Macro-ovalocytes + hypersegmented neutrophils
• AIHA: Spherocytes + polychromasia + positive Coombs test

Evaluation

ED Indications for Ordering a Peripheral Blood Smear

• Unexplained anemia, especially when etiology is unclear from CBC indices alone
• Thrombocytopenia — to rule out pseudothrombocytopenia (platelet clumps) and evaluate for schistocytes (TMA)
• Suspected TTP, HUS, DIC, or HELLP — schistocyte identification is essential for TMA diagnosis and should not be delayed
• Suspected acute leukemia — blasts on smear → emergent hematology consultation
• Suspected malaria or babesiosis — thick and thin smear remain the gold standard
• Pancytopenia or leukoerythroblastic picture
• Unexplained leukocytosis with concern for malignancy
• Clinical suspicion of sickle cell crisis in an undiagnosed patient
• Fever in a returning traveler

How to Order

• Order "peripheral blood smear with pathologist review" or "manual differential with smear" — terminology varies by institution
• Specify clinical question (e.g. "evaluate for schistocytes" or "rule out blasts") to guide the reviewing pathologist or technologist
• If malaria is suspected, order "thick and thin blood smears for malaria parasites" — standard Wright-stained smear may not be optimized for parasite detection

Interpreting Results

• Smear results should always be interpreted alongside the CBC, clinical context, and other labs
• A single negative malaria smear does not exclude infection; repeat every 12–24 hours for 3 sets if clinical suspicion is high
• Absence of schistocytes does not definitively exclude TMA; a threshold of ≥ 1% schistocytes supports the diagnosis, but clinical judgment should prevail
• Smear artifacts (echinocyte formation from alkaline stain, stomatocytes from acidic stain, crenated cells from drying) should be recognized and not misinterpreted

Management

Management is directed by the underlying diagnosis suggested or confirmed by the PBS:

• Schistocytes identified + thrombocytopenia:
  • Assume TMA until proven otherwise — calculate PLASMIC score if TTP is suspected
  • Send ADAMTS13 activity level (results take days; do not delay treatment)
  • Emergent hematology consultation for TTP (plasma exchange is lifesaving and should not be delayed)
  • Do NOT transfuse platelets in suspected TTP (may worsen microvascular thrombosis)
  • Evaluate for DIC (PT/INR, PTT, fibrinogen, D-dimer) — abnormal coagulation studies favor DIC over TTP
• Blasts on smear:
  • Emergent hematology/oncology consultation
  • Evaluate for tumor lysis syndrome (potassium, phosphorus, uric acid, calcium, LDH, creatinine)
  • If WBC > 100,000/μL with blasts → concern for leukostasis; may require emergent leukapheresis
  • Do not delay flow cytometry and bone marrow biopsy
• Malaria parasites:
  • Calculate percent parasitemia from thin smear
  • > 5% parasitemia or any evidence of severe malaria (altered mentation, renal failure, severe anemia, ARDS) → IV artesunate
  • Infectious disease consultation
• Spherocytes + hemolysis labs:
  • Send direct Coombs (direct antiglobulin test)
  • If positive → autoimmune hemolytic anemia; consider steroids
  • If negative → consider hereditary spherocytosis, delayed transfusion reaction
• Leukoerythroblastic picture (tear-drop cells + nucleated RBCs + immature granulocytes):
  • Suggests marrow infiltration (myelofibrosis, metastatic disease, granulomatous disease)
  • Hematology consultation for bone marrow biopsy

Disposition

• Admit:
  • Schistocytes with thrombocytopenia (suspected TMA) — often to ICU; emergent plasma exchange for TTP
  • Blasts on peripheral smear (suspected acute leukemia)
  • Severe parasitemia (malaria > 5% or any signs of severe malaria)
  • Symptomatic anemia requiring transfusion with unclear etiology
  • Pancytopenia with fever
• Discharge with urgent follow-up may be appropriate for:
  • Mild morphologic abnormalities (e.g. target cells, mild poikilocytosis) with stable CBC and known chronic condition
  • Confirmed pseudothrombocytopenia with normal citrate platelet count
  • Stable atypical lymphocytosis consistent with viral syndrome
• Communicate critical smear findings (blasts, schistocytes, parasites) directly with the consulting service — do not rely solely on electronic result notification

See Also

• CBC
• Anemia
• TTP
• Hemolytic Uremic Syndrome
• DIC
• Leukemia
• Neutropenic fever
• Malaria
• Babesiosis
• Sickle Cell Disease
• HELLP Syndrome
• Thrombocytopenia
• Pancytopenia

External Links

• Peripheral Blood Smear - Clinical Methods (NCBI Bookshelf)
• Peripheral Blood Film — A Review (Ann Ib Postgrad Med 2014)
• ICSH recommendations for schistocyte identification and quantitation (Int J Lab Hematol 2012)
• Diagnosis from the blood smear (N Engl J Med 2005)
• Utility of PBS review for identifying specimens for flow cytometry (Int J Lab Hematol 2017)